This invention relates to multi-cavity valve-gated injection molding, and more particularly to a nozzle seal to be used in the same.
In a typical multi-cavity arrangement, a number of heaters are geometrically spaced from the molding machine and the molten plastic material or melt flows from the molding machine through radially extending hot runner passages, down through the heater casts, past the valve pins and into the cavities. The lower tip of each valve pin is forcibly seated in a gate in the respective cavity plate and, as is well known in the art, the most critical operation of the mechanism is in the gate area where malfunctions may cause costly shut-downs of the machine or imperfections in the molded product. The lower nozzle portion of the heater cast is separated from the cavity plate by an air space which provides thermal insulation between the hot nozzle and the cooler cavity plate. In the past, this air space has at least partially filled with plastic material which similarly serves as an insulator. In molding products where space between cavities is not a problem, a locating bushing is used to locate the nozzle portion of the heater cast relative to the cavity plate to prevent lateral movement therebetween due to thermal expansion and contraction. However, in certain applications where the heater cast has several nozzles protruding from it and it is necessary to arrange the gates as close together as possible, it is desirable to replace the locating bushings by smaller insulation bushings and allow for lateral and vertical movement of the nozzle portion in the air space relative to the cavity plate due to thermal expansion and contraction.
However, it has been found that in molding certain plastic materials with high temperature characteristics such as nylon, polycarbonate, ABS, and acrylic, the valve pin does not always operate satisfactorily and particularly the tip of the valve pin does not seat satisfactorily in the gate. Not only is this troublesome and costly in itself, but it has the effect of requiring greater seating pressure on the valve pin, thereby placing greater stress on the valve pin operating mechanism which decreases the expected operating life of the mechanism as a result of increased water. It has been found that these problems may be substantially reduced by providing for increase heat transfer to the immediate gate area, while avoiding the problem of relative movement between the lower nozzle portion of the heater cast and the cavity plate due to thermal expansion and contraction.